Depletion of petroleum resources have been feared for a long period. Meanwhile, research efforts have been made for technologies of manufacturing various hydrocarbon oils such as naphtha, kerosene and gas oil by exploiting natural gas, coal and alternative carbon sources such as biomass in order to reduce the degree of dependence upon petroleum resources. Of such technologies, those of GTL processes have got to a practically feasible level. GTL plants of commercially feasible scales have already been constructed and started operations in areas abundantly producing natural gas. There are trends of constructing similar plants.
A GTL process is for manufacturing various petroleum products such as naphtha, kerosene and gas oil by reforming natural gas containing methane (CH4) as main component to produce synthesis gas containing hydrogen (H2) and carbon monoxide (CO) as main components, producing so-called Fischer-Tropsch oil (FT oil) that is a mixture of various hydrocarbon oils containing heavy hydrocarbons by way of Fischer-Tropsch synthesis (FT synthesis) using the synthesis gas as raw material, and upgrading and refining the obtained FT oil. This, a GTL process is roughly a three step process including a synthesis gas production step (reforming step), a Fischer-Tropsch oil manufacturing step (FT step) and an upgrading step (UG step).
When producing synthesis gas, firstly the sulfur compounds contained in the natural gas that is to be consumed as raw material are desulfurized in a desulfurization apparatus. Subsequently, steam and/or carbon dioxide are added to the desulfurized natural gas, which desulfurized natural gas is then introduced into a synthesis gas production apparatus (to be also referred to as “reformer” hereinafter) and heated. As a result, the reforming reaction proceeds in the reformer by the effect of the reforming catalyst filled in the reformer. Thus, synthesis gas is produced in this way. While the steam reforming method using steam is mainly employed for a reforming reaction, the carbon dioxide reforming method using carbon dioxide has been put to practical use in recent years. The use of the carbon dioxide reforming method provides an advantage that the synthesis gas production step can be conducted efficiently at low cost because the carbon dioxide contained in natural gas does not need to be separated and removed before the reforming reaction. Additionally, more carbon dioxide can be used as resource because the unreacted carbon dioxide and the generated carbon dioxide that are contained in the produced synthesis gas can be separated, collected and recycled to the synthesis gas production step so as to be reutilized for the carbon dioxide reforming method
In the synthesis gas production step, the carbon dioxide contained in the synthesis gas produced by reforming of natural gas is separated and collected by means of chemical absorption method in a carbon dioxide separation/collection unit, and the collected carbon dioxide is recycled to the synthesis gas production apparatus as source gas for reforming natural gas. In the process of separating and collecting carbon dioxide by means of a chemical absorption method, the absorbing solution stored in the carbon dioxide separation/collection unit absorbs carbon dioxide. Normally, a stainless steel material that contains nickel has been used as filling material for the absorption tower and the regeneration tower of a carbon dioxide separation/collection unit. Also normally, an aqueous solution that contains primary amines has been used as absorbing solution. However, the inventors of the present invention have found that an aqueous solution containing primary amines that absorbs carbon dioxide corrodes the filling material of carbon dioxide separation/collection unit, which is a nickel-containing stainless steel material. The manufactured synthesis gas contains carbon monoxide, which reacts with the nickel component dissolved into the aqueous solution as a result of corrosion and the filling material in the carbon dioxide separation/collection unit to produce nickel carbonyl.
The nickel carbonyl produced in the carbon dioxide separation/collection unit in this way is then supplied to the reformer along with the recycled carbon dioxide as source gas for reforming natural gas. Then, the nickel carbonyl supplied to the reformer adheres onto the reforming catalyst filled in the reformer and then carbon as a result of a side reaction of the reforming reaction is deposited and accumulated. Then, degradation of the catalyst activity of the reforming catalyst is feared. Additionally, the supplied nickel carbonyl can be decomposed to produce nickel metal, and then the nickel metal can adhere to the rotating machines such as compressors and the heat exchangers installed in the recycle line. Then, damages to those machines are feared because such damages can adversely affect the stable and long term operation of the machines.